Scored piston

Scored piston

Fig 7.27

Scoring is an advanced stage of scuffing (Figure 7.27). As well as the original cause of scuffing, continued operation of the engine with scuffed pistons and cylinders will generate excessive heat and friction. This will exaggerate the problem, so that a large area of the piston becomes scored.
Deep scoring in a cylinder can be the result of a mechanical failure, such as a broken piston ring, a broken circlip, or a loose piston pin. This type of damage could be confined to only one cylinder and the cause is easy to see.

Abrasion is a type of light scoring that could be caused by dust and dirt entering the engine through a faulty or badly serviced air cleaner.
There will be abrasive wear to the valve sterns and guides as well as to the pistons, rings and cylinders. The scoring will not be deep, but will be in the form of a mat finish of very fine score marks covering all the rubbing surfaces. All the cylinders would be affected.

Fig 7.28

Seized piston
 

With a seized piston, the entire thrust surfaces would be marked (Figure 7.28). A likely cause is insufficient lubrication. If this is the correct diagnosis, the bearings and journals will also be affected and the engine oil will be contaminated.
Other factors that could cause a breakdown in the oil film between the piston and cylinder must also be considered, such as cooling-system problems, or blowby due to sticking rings.

·              Scuffing, scoring and seizing are conditions that are closely related. In some cases, they could be three stages of damage from the same initial cause.

Fig 7.29

Corrosion can affect a piston as shown in Figure 7.29. This can be the result of coolant leaking into the combustion chamber and finding its way down past the piston rings. This would affect cylinder-wall lubrication and could cause scuffing.
A coolant leak can contaminate the engine oil and form sludge in the oil pan. This will further affect cylinder lubrication. Blowby can also contaminate the engine oil, and make the additives less effective.

Blowby will occur if the piston rings do not seal properly. The hot gases that blow past the piston will increase the temperature of the piston, piston rings and cylinder walls.

The tension of the piston rings could be lost because of overheating, and so the problem of blowby increased. Where the engine is continued in operation, this can destroy the piston lands and rings as shown in Figure 7.30.

Fig 7.30

Piston damage

Figure 7.31 shows damage to the piston-pin boss area of a piston. This has been caused by a loose circlip. Circlips must be a good fit in their grooves and should be renewed whenever an engine is dismantled.
Misalignment of the connecting rod can cause side thrusts on the piston pin which are transmitted to the circlip. This could break the circlip, or damage the piston. If the circlip breaks, particles could score the piston and cylinder, or jam the piston rings, to eventually produce damage such as that illustrated. 

Fig 7.31

Continued

See General engine service>>>>>>>>>>

Connecting-rod aligner

Fig 7.25

This is a fixture that consists of a vertical faceplate, with a horizontal mandrel. The big end of the connecting rod is mounted on the mandrel and the rod alignment is checked against the faceplate.
A specially-shaped jig or V-block is used — the V of the jig is shaped to sit on the piston pin. Small dowels that extend from the jig are checked against the faceplate to determine rod alignment (Figure 7.25).

Checking alignment

Install the piston pin in the connecting rod, mount the rod on the mandrel of the aligner, and place the V of the jig over the piston pin.
Slide the rod along the mandrel until the dowels of the jig just touch the surface of the faceplate. If the dowels do not touch the plate, then the rod is misaligned.
The rod should be checked for bend and then for twist. To do this, the jig is mounted in two different positions on the piston pin:
1. with the dowels vertical. This will check for bend in the rod. If the rod is straight, both vertical dowels will be in contact with the faceplate (Figure 7.25(a)). If only one dowel contacts the faceplate, then the rod is bent.
2. with the dowels horizontal. This will check for twist in the rod. If the rod is true, and not twisted, both horizontal dowels will be in contact with the faceplate (Figure 7.25(b)). If only one dowel contacts the faceplate, then the rod is twisted.

·        The allowable misalignment, for both bend and twist is around 0.10mm. This is checked with feeler gauges between the dowels and the faceplate.

Straightening connecting rods
Connecting rods that are slightly misaligned can be straightened in a special jig designed for this purpose. A press or bending bar can also be used.
When straightening a connecting rod, it should be bent a little beyond the point where it is straight and then bent back to obtain correct alignment. This relieves the stress in the rod and it is less likely to tend to take up a permanent set and, for this reason, doubtful or badly misaligned rods should be replaced.

Analyzing piston, ring and cylinder problems
 

Engine noises, loss of engine power and exhaust smoke are indications that there could be problems with pistons and cylinders. While these could be due to wear in a vehicle with high kilometres, excessive wear or damage needs to be investigated.
The cause of a fault must be located and corrected so that the problem does not occur again. For damaged pistons and cylinders, do not just blame the part that failed, but consider the following as possible causes:

1. Operating conditions of the vehicle

2. Lubrication
3. Cooling
4. Temperature

5. Connecting-rod alignment

6. Other possible cause.

Following are descriptions of a number of piston faults that are not the result of normal wear. These are shown as separate faults, but some are a more advanced stage of one of the other faults, The cylinders in which the pistons operate will also be damaged, but the piston will show more damage because it is made of softer material.
When diagnosing a problem, consider whether it is related to the whole engine, or whether it is confined to only one cylinder. Also, consider whether the problem is related to the piston, the piston rings, or the cylinder, although one usually affects the others.

Scuffed pistons

Fig 7.26

Scuffing is a light abrasion on the thrust sides of the piston (Figure 7.26). It is caused by heat. It occurs when two surfaces are rubbing together and the temperature rises until melting point is reached. Small particles of metal then weld together, leaving small deposits on one surface and small holes in the other.
When scuffing starts, the damage can spread, being aggravated by the rough surfaces until an area of the piston is affected as shown.
The following should be checked for possible causes of scuffing or other piston failure:

I. The water-jackets for restrictions or deposits that could cause hot spots on the cylinder walls or distortion of the cylinder.
2. The lubrication system for correct pressure and type of oil.

3. The piston for size and clearance.
4. The connecting rod for alignment. 

Continued

See scored pistons>>>>>>>>>>>>>>>>>>

Cont. Connecting rod beaing installation

Fig 7.20

When bearings are to be installed, make sure that hand, workbench, tools and all engine parts are clean. Keep new bearings wrapped until ready to be installed and then handle them carefully. To avoid problems, wipe each bearing with a fresh piece of cleaning rag and install it into a clean bore (Figure 7.20).
The bearings have locating tangs that fit into notches in the connecting rod and cap. Make sure that the tangs enter the notches correctly. See the comments about bearing spread and crush that follow.
Bearing clearance cannot be adjusted. Any attempt to correct bearing clearance by filing the connecting-rod cap will destroy the original relationship between the cap and the rod. This will cause early bearing failure.

Bearing spread and crush

Fig 7.21

Bearing inserts are provided with spread and crush (Figure 7.21). With spread, the ends of the bearing are slightly wider than the bore of the rod or cap into which the bearing fits. When the bearing is being pushed into the cap or into the connecting rod, it will snap into place and the spread of the bearing will keep it there.
Crush is caused because the edge of the bearing insert stands above the parting face of the connecting rod or cap. This additional height is ‘crushed’ when the cap is installed and the bolts tightened. Crushing pushes the inserts into the bore in the connecting rod and cap. This ensures that the backs of the inserts are in snug contact with the bore. Figure 7.22 shows

Fig 7.22

the top half of a bearing correctly installed in a connecting rod.
Crush is a general term as described, but it can also refer to the distance that the edge of the bearing stands above the parting face.

·       New hearings will have the correct crush and must not be

tampered with in any way.

Checking connecting-rod bearings

Automotive engines are fitted with precision-insert bearings that are accurately finished to size and require no adjustment. In fact, adjustment should never be attempted as this will ruin the bearing.
The clearance of a connecting-rod bearing can be found by using a telescopic gauge to measure the bearing and a micrometer to measure the journal. However, the easiest way is with Plastigage.

Plastigage

Fig 7.23

Plastigage is a plastic material that comes in strips and flattens under pressure. It is used to check the clearance between bearings and shafts.
When used to check the clearance of a connecting- rod bearing, a strip of the material is placed in the bearing in the cap. The cap is installed and the nuts are tightened to the correct torque. The bearing cap is then removed and the flattened Plastigage is measured (Figure 7.23). If the Plastigage is flattened only a little, then there is a large-clearance, and vice versa.
A printed scale is supplied with the Plastigage. The scale has bands of different widths which are marked to show the bearing clearance — for example, 0.02 mm, 0.03 mm etc. The scale is placed against the flattened Plastigage to check its width as shown in the illustration.

The scale measures the width of the flattened Plastigage, but shows this as its thickness, which is the clearance between the bearing and the shaft.

Using Plastigage

1. Wipe the crank-pin and bearing clean of oil.

2. Position the crank-pin of the bearing to be checked about 30° before BDC, because this is where the bearing clearance is likely to be greatest.
3. Place a small strip of Plastigage in the centre of the bearing and install the bearing cap on the connecting rod.
4. Remove the bearing cap and check the thickness of the flattened Plastigage against the scale.

·   Do not move the crankshaft while the cap nuts are tight,- this would disturb the Plastigage and give an inaccurate measurement.

Connecting-rod alignment
 

Fig 7.24

The connecting rod must be in alignment — that is, there should be no twist or bend in the rod. Any misalignment will cause side thrusts on the piston and irregular loading of the bearing.

Figure 7.24 shows the effects of a misaligned connecting rod. The heavy loading at points A and B on the bearing would cause bearing failure at these points.
The heavy-pressure spots C and D on the piston will cause heavy wear and possibly scoring of the piston and cylinder wall. A basic inspection check is to look for uneven wear or shiny spots on the pistons, which will indicate misalignment. If this condition is found, the connecting rod should be checked and as necessary, straightened or replaced.

Continued

See connecting rod aligner>>>>>>>>>>

Checking rings on a piston

Fig 7.12

Rings are checked in their grooves to make sure that they are free and also that the groove is the right depth. The grooves must be clean, with all carbon removed. A groove-cleaning tool or a broken ring is used for this purpose.
Place the face of the new ring in the groove, as shown in Figure 7.12, and roll it around the piston. it should be free in all parts of the groove. If the ring is tight in any spot, the groove should be cleaned and the ring tried again.

Check the ring-groove depth. With the face of the ring held into its groove, place a steel rule across the lands.

There should be clearance between the rule and the ring. Average specifications are around 0.3 mm. Some replacement rings might require deeper grooves, and the instructions supplied by the manufacturer should be closely followed.

Fig 7.13

Ring side clearance
The side clearance of the ring in the groove is checked with a feeler gauge (Figure 7.13). This can be done by holding the ring in its groove, or with the ring installed on the piston. Average specifications for side clearance are 0.035 mm for petrol engines and 0.05 mm for diesel engines.

Installing rings on a piston
The steel rails and separators of segmental rings are carefully wound into the groove in the piston, one piece at a time, making sure that they are correctly seated (Figure 7.14). The ring can be turned in its groove if it is correctly installed.

Fig 7.14

Cast iron compression rings can be installed in a similar way, but care must be taken that the

rings are not distorted, The best method for compression rings is with a ring-expanding tool (Figure 7.15). This supports the ring while it is being spread. The ring should be expanded just enough to allow it to pass over the piston.

Compression rings
A compression ring with a tapered face or counter-bore must be installed with the correct side of the ring to the top, as shown in Figure 7.15. If a ring is fitted upside down, its action will be reversed. A scraper ring, for example, would carry oil up the cylinder wall into the

Fig 7.15

combustion chamber. This would cause high oil consumption and associated problems.

Ring gaps
When the rings are installed in their grooves, the ring gaps should be staggered so that all the gaps are not in line. The gaps are kept away from the thrust sides of the piston. Figure

7.16 shows a set of rings and how the gaps are arranged.

Installing pistons in cyLinders

Fig 7.16

A piston-ring compressor is used to install the piston in the cylinder (Figure 7. 17). The compressor clamps around the piston rings and compresses them into their grooves so that they will enter the cylinder bore without being damaged.
Following are related points:
1. Oil the piston and rings and stagger the ring gaps.
2. Fit a piece of plastic tube over the connecting-rod bolts so that they will not scratch the crank-pin journal (Figure 7.18).

3. Fit the ring compressor over the piston and tighten it firmly so that the rings are compressed.

4. Place the piston and connecting rod in the cylinder, with the front mark of the piston towards the front of the engine.
5. Insert the piston using light even taps with a hammer handle on the head of the piston. Only light taps will be needed if a good compressor is used and operated correctly. Heavy

Fig 7.17

blows will damage the piston and break the rings.
6. Hold the compressor firmly against the surface of the cylinder block so that there is no gap for the rings to escape.

·       The piston rings must be compressed to enter the cylinder. A ring that is not held by the compressor is likely to be broken.

Installing connecting-rod bearings
New precision connecting-rod bearings are required if the old ones are defective or have worn so much that clearances are excessive. New bearings will also be required if the crankpin journals have worn out-of- round or tapered to the extent that

Fig 7.18

the crankshaft has to be reground. In this case, the journals will be finished undersize and undersize bearings would be used. Undersize bearings would have some form of identification to show that they were not standard bearings (Figure 7.19).

Crank-pin journals should be checked with a micrometer for taper or eccentricity. If the journals are out-of-round or tapered more than 0.04 mm (example) the crankshaft must be replaced or the crank-pins reground.

·       For bearing faults, refer to the section ‘Analyzing bearing failures’ in previous posts

Fig 7.19

showing typical engine bearing problems.

Continued

See Installing connecting rod bearings>>>>>>>>>>

Piston measurement and clearance

A piston must be in good condition if it is to be reinstalled in the engine. This means that it must have no cracks, scores or scratches. It must be the correct size and have the correct clearance in the cylinder.
The piston is measured with an outside micrometer or vernier calipers. The cylinder is measured with an inside micrometer and the two sets of measurements are compared to find the piston clearance.

Measuring the piston

Fig 7.7

Using a micrometer, measure the piston skirt across the thrust faces as shown in Figure 7.7. In most cases, the measurement is taken at a point which is approximately one-third of the skirt height. This is the nominal diameter of the piston and should be the greatest dimension.

Other measurements can be taken as a check and to make sure that the piston has not collapsed. A collapsed piston will have a reduced diameter at the lower end of the skirt.

Fig 7.8

The engine manufacturer’s service manual should be consulted for piston dimensions and clearances. It will also have information on how to measure the piston. Some pistons are cam-ground, some have a tapered skirt and some are slightly barrel-shaped. All pistons are not measured in the same way.

Figure 7.8 is an example of piston measurement. It shows the places on a piston where measurements would be taken. These are as follows:

BB is the nominal piston diameter
AA is the reduced diameter after cam grinding
CC is the diameter at the top of the skirt
DD is the diameter at the bottom of the skirt
EE is the reduced diameter at the lands.

· These measurements enable the piston size, cam grinding, skirt taper and land
relief to be determined.

Checking the piston in its cylinder

Fig 7.9

The fit of the piston in the cylinder can be checked with a feeler-gauge strip. The following is a typical procedure.
Place the piston in the cylinder upside down with the feeler strip, lightly oiled, placed 900 from the piston-pin holes. This is the greatest piston diameter. The feeler strip should extend the full length of the piston. Different thicknesses can be tried to determine the clearance.

A refinement of this procedure, which is more exact, requires the use of a spring scale. The force required to pull the feeler strip from between the piston and cylinder is measured (Figure 7.9). The following is an example: a strip of feeler 12 mm wide and 0.04 mm thick should be able to be withdrawn from between the piston and the cylinder wall with a pull of 20 to 40 newtons. If the feeler strip pulls out too easily, the fit is too loose, if it is too hard to pull, the fit is too tight.

Fitting piston rings
Replacement piston rings are supplied as a package kit to suit the particular engine being repaired. They can be obtained in various over sizes to suit oversize cylinder bores.
Piston rings are supplied for a particular bore diameter. Their ends should not be filed to fit them to smaller bores because they will become oval-shaped when installed in the cylinder, if the cylinder has been re-bored or made oversize by honing, it will require oversize rings; if not, standard-sized rings must be used, irrespective of the wear that might have occurred to the upper portion of the cylinder.

Fig 7.1

Checking rings in the cylinder bore
New piston rings should be checked in the cylinder to make sure that they are correct for the bore size (Figure 7.10).

A quick check can be made with the ring near the top of the bore to make sure that it has a gap. However, before a measurement is actually taken, the ring is pushed down the cylinder with the head of a piston (Figure 7.11). This makes sure that the ring is sitting squarely in the bore. Worn bores will be tapered, and so the ring should be pushed down to the part of the bore that is least worn. This will be below the lower limit of ring travel.
Check the gap between the ends of the ring with feeler gauges. A rule of thumb is that the

Fig 7.11

gap clearance should be 0.03 mm for each 10 mm of cylinder diameter, measured in the unworn part of the bore. 
 

·  The ring must not be measured at the top of the bore because its gap will close as it is moved towards the bottom of the cylinder. Without a gap, the ring will break or cause scuffing of the cylinder walls.

Continued

See piston rings checking>>>>>>

Piston, connecting rod and bearing service

Satisfactory engine performance cannot be obtained unless the piston and connecting-rod assembly are in good condition.
The piston must have correct clearance in the cylinder to avoid piston slap, the compression rings must seal to prevent pressure loss during compression and combustion, the oil ring must control the oil on the cylinder walls, and the piston pin must be the correct fit to prevent noise.
The connecting-rod bearings must not be damaged or worn, and the crank-pin journals must be in good condition to prevent oil loss and noise.

Dismantling the piston assembly
The following are points that relate to dismantling a piston and connecting-rod assembly:
1. Remove the piston rings by winding them out of the grooves in the piston. A thick feeler gauge used under the rings will help.
2. Push the halves of the bearing from the connecting rod and cap. Check for difference between the upper and lower halves. The upper half of the bearing may have an oil hole.
3. Check the piston pin by trying to move the connecting rod in relation to the piston.
4. If the piston is to be removed from the connecting rod, check for markings that identify the front of the piston in relation to the connecting rod.
5. Separate the piston from the connecting rod by removing the piston pin. Depending on the type, the pin may have to be tapped or pressed out.
6. Temporarily install the cap on the connecting rod so that the bearing and all the other parts of the assembly are together.
7. Clean the various parts and check for wear.

·           All parts must be kept to their original cylinders — mark or label as necessary.

Removing and replacing piston pins

Fig 7.1

Normally, the piston and connecting rod are only separated if there seems to be looseness between the piston and the rod. To check the fit of the piston pin, hold the piston firmly and try to rock the connecting rod from side to side (Figure 7.1). There should be little or no movement.

If there is obvious movement then the piston pin, the pin holes in the piston, or the bush in the connecting rod (where fitted) will be worn. The piston should be dismantled from the connecting rod so that the parts can be examined.

Fig 7.2

Floating piston pins
Removing and installing a floating piston pin is shown in Figure 7.2. This has circlips that have to be removed from the piston before the piston pin can be removed.
The piston pin can be tapped out, using a light hammer and a punch. The piston must be suitably supported to prevent it from being damaged and the punch should have a pilot that fits into the hollow piston pin.

·   If the piston pin is tight, the piston should be heated before the pin is 
removed.

When dismantled, the bush in the small end of the connecting rod should be checked for wear, as well as the piston pin and the pin holes in the piston.
In some cases, a worn bush in the eye of a connecting rod can be replaced by pressing out the old bush and pressing in a new one. Figure 7.3 shows this operation for a connecting rod of a diesel engine. This is a rugged connecting rod with a relatively thick bush. The eye of the rod must be supported and a suitable pressing tool used. The bush has

Fig 7.3

an oil hole that must be aligned with the hole in the rod.
Before installing the piston pin, the piston should be heated so that the pin can be installed without damage. New circlips should be used.

Figure 7.2 Floating piston pin
(a) the circlips are removed with a small
screwdriver (b) the piston pin is installed to a heated piston (c) new circlips are fitted

Press-fit piston pins
A pressing tool and a supporting tool are needed when removing or installing piston pins that are a press fit.

This type of pin is free to move in the piston bosses, but is a press fit in the small end of the connecting rod.
Figure 7.4 shows how the piston is being supported while the piston pin is being pressed out of the connecting rod. The pin must not be driven out. Before removing a piston pin, note its exact position in the connecting rod so that it can be replaced in the same position.

Fig 7.4

Press-fit piston pins have an interference fit of about 0.02 mm in the eye of the connecting rod. When

a pin is to be installed, the eye of the rod can be heated carefully on a hotplate so that the piston pin can be easily fitted.
The pressing tool and support arc used to press the pin back into the connecting rod. This must be done quickly so that the pin is in its correct position before the connecting rod cools and contracts.

Heating the piston or rod
To heat the piston, place it in a container of water and heat the water to almost boiling point (Figure 7.5). This will expand the piston and make the piston pin easier to remove. This can be done before dismantling and also before reassembling.

Fig 7.5

An alternative method, only for press-fit piston pins, is to carefully heat the end of the connecting rod on a hotplate. This will expand the eye of the connecting rod and allow the piston pin to be fitted.

·   Direct flame should not he applied to a connecting rod.

Piston-pin fit
A piston pin that is fitted too tightly could cause distortion, while a pin that is too loose could cause a knock.
Engine manufacturers specify clearances, such as 0.002—0.008 mm between the piston pin and the piston, with a wear limit of 0.02 mm. These are very small clearances. The small clearances are possible by having the piston pin a selective fit in the piston. The piston pin and the hole in the piston have extremely fine surface-finishes and this reduces wear.

Types of piston pins fit

Fig 7.6

During manufacture of parts, special gauges are used and these are capable of measuring to an accuracy of 0.0002 mm. In service workshops, where clearances have to be checked, such accuracy is not needed and micrometers, calipers and dial gauges are used.
There are also ways of describing the fits between parts — three descriptions of the fit of piston pins are illustrated in Figure 7.6. In order of tightness, these are hand-push fit, thumb-push fit and finger-push fit. There are other descriptions that can be used, such as tap fit, light-tap fit and drive fit.

·      While these fits are not measurements, they are useful as workshop

descriptions. 

Continued

See piston measurements and clearance>>>>>>

Connecting rods

Fig 6.23

The connecting rod must be very strong and rigid, but it must also be as light as possible. Connecting rods are highly stressed, being subjected to stretching, compression and bending. They are made as a forging of H-section, because this shape provides greater strength than a solid rod of the same mass (Figure 6.23).

The crank-pin end is the larger end of the connecting rod and is often referred to as the big end. The piston end is much smaller and so is referred to as the small end, or sometimes as the little end. The big end of the connecting rod has a bearing cap that enables it to be installed on the crank-pin. The big end also carries the two halves of the bearing.

Fig 6.24

Connecting-rod features
To maintain engine balance, all the connecting rods in an engine are matched for mass. The cylinder number is stamped on most rods and caps so that they will not become mixed up when the engine is dismantled (Figure 6.24).
During the manufacture of most connecting rods, the parting faces of both the bearing cap and the connecting rod are machined as flat surfaces. The cap is bolted to the rod, and the

bore for the bearing is then finished to an accurate size.
Another process uses a fracture method. The connecting rod is made as a single piece and the cap is then broken away from the rod. The cap is reassembled to the rod and the bore for the bearing is machined. With this process, the parting faces of the cap and the connecting rod are irregular, but they fit together so closely that the parting line can barely be seen.

Connecting-rod bolts
Some connecting rods have cap bolts with nuts, similar to those in Figure 6.23. The bolts are a neat fit in the connecting rod and are not usually removed unless they are to be replaced. Other connecting rods have cap bolts that are threaded into the connecting rod.
Some bolts have their shank reduced in diameter so that it is less than the thread and this actually increases their strength. In a normal bolt, the root of the thread is the smallest diameter and so takes the greatest stress. When the shank of a bolt is reduced in size, the stress is distributed over a different part of the bolt and so prevents failure at the thread.

·           The cap bolts are subjected to a tensile stress (stretching) caused by the piston inertia

at the top of the stroke.

Forces in a connecting-rod assembly

Fig 6.24

Forces in a connecting-rod assembly

The connecting-rod and piston assembly are subjected to various forces. These are caused by combustion pressure, piston inertia and centrifugal force.
An example of the way in which inertia forces act on the various parts of the assembly can be seen in Figure 6.25. The piston in the illustration is almost at the end of an upstroke (exhaust stroke) and at TDC will produce an inertia force that will be transmitted to other parts of the assembly.

On the down-stroke, when the piston reaches BDC, the direction of most of the forces due to inertia will be the reverse of those shown.
The piston provides an inertia force at TDC and BDC where the piston changes its stroke. In effect, the piston tries to keep moving but is held by the connecting rod. This places the connecting rod in tension when the piston is at TDC and in compression when the piston is at BDC.  

Figure 6.25 Effects of inertia of the piston at the end of the exhaust stroke — the assembly is for a diesel engine

1 piston inertia, 2 toad on upper half of small-end bush,
3 connecting rod in tension, 4 bolts in tension, 5 toad on
tower half of big-end bearing, 6 toad on upper half of main
bearing

There are many forces acting at different times. On the compression stroke, compression pressure will have an influence on the forces, as will combustion pressure during the power stroke. The effects of centrifugal force, due to crankshaft rotation, will increase with engine speed.
The assembly shown in Figure 6.25 is for a larger diesel engine. The parts are bigger and have a greater mass than those of a petrol engine, so greater stress can be created. However, the engine rotational speeds (and piston speeds) will be much lower than those of a petrol engine.

Connecting-rod bearings
Connecting-rod bearings are split-sleeve bearings of the precision-insert type. One half of the bearing is carried in the connecting rod and the other half in the connecting-rod cap.
Various points relating to crankshaft bearings were discussed in last posts, and there is more information related to bearings in the posts that follows.

Continued

See piston, Connecting rod and bearing service>>>>>>>>>>>>>>>>>>